Implement substring search and weighted sampling

Two coding questions were asked in the onsite. 1. **Substring search**: Given two strings `text` and `pattern`, return the starting index of the first occurrence of `pattern` in `text`, or `-1` if `pattern` does not occur. Assume the input can be large enough that a purely naive `O(n * m)` scan may be too slow. 2. **Weighted random picker**: Design a class initialized with an array of positive integer weights. Implement a method `pickIndex()` that returns index `i` with probability `weights[i] / sum(weights)`. Follow-up: briefly discuss how the design would change if weights were updated frequently.